1. Field of the Invention
This invention relates to video data compression. More particular, this invention relates to the real time recording of video data as compressed video data.
2. Description of the Prior Art
Data compression is utilised in the still image field. The proposed JPEG standard provides a non real time image data compression technique for use in situations such as the compression of still images by and for storage within general purpose computers. The JPEG technique utilises quantization as One step in the compression process. The quantization step width applied may be changed to change the degree of compression achieved. Using a Small quantization step width will increase the length of time needed to encode the data (limited rate data channels within a computer) and the amount of storage space needed to store the compressed image. Conversely, using a large quantization step width will decrease the length of time needed to encode the data and the amount of storage space needed to store the compressed image. The asynchronous, non real time nature of the JPEG approach means that such variation is not an issue of importance.
It is also known to utilise real time compression systems in conjunction with storage applications in order to increase the storage capacity. A requirement in such systems is the achieving of a constant output bit rate compatible with the storage medium used. This approach results in a variable picture quality depending upon the image information content. Furthermore, since these systems have to achieve an acceptable picture quality with the most difficult of images, they produce an image quality in excess of that genuinely required for the majority of pictures with a consequential reduction in storage capacity.
The constant data rate approach is imposed on convention compression systems due to the nature of the storage media that are generally used. In particular, storage media such as magnetic tapes, hard-discs and magneto optic discs have mechanical inertia such that they are only able to deal with a constant data rate when operating close to their peak performance. Thus, for images with a small amount of information content, the image quality exceeds the specification (and is likely to be lossless) and for images with a large amount of information, the image quality can be below specification.